The screen of a LCD device generally comprises many liquid crystal cells arranged in columns and rows, forming a pixel array to display images. In each pixel, the orientation of liquid crystal molecules can be controlled by the applied voltage. Since the intensity of light passing through a liquid crystal cell depends sensitively on the orientation of the liquid crystal molecule, the pixel array can therefore display images by applying voltage signals in accordance with input video signals. However, due to its inherent limitations, it requires a relatively long response time for a liquid crystal molecule in a certain orientation to be changed into another orientation as the applied electric field is changed accordingly. This response time is determined by the inherent property of the liquid crystal molecule, such as viscosity, dielectric and elastic constants. On the other hand, the response time also depends on the design of LCD panel, such as the thickness of the gap between two electrodes. For a twisted-nematic mode liquid crystal, the typical rise time is about 20–80 ms, and the fall time is about 20-30 ms. However, this time scale is still longer than a typical frame interval (typically 16.67 ms). This means that the liquid crystal molecules in each pixel cannot reach the desired orientation during one frame interval, so that desired brightness of each pixel cannot be reached, thus resulting in afterimage and blurred image when displaying a high-speed moving object.
Except looking for faster liquid crystal materials, the problem of afterimage caused by slow response time can also be overcome by suitable driving method for the LCD device. In general, the problem of afterimage can be effectively reduced by a gray signal modulator, which modulates the input gray signal and applies the modulated gray signal to the liquid crystal cell, thereby obtaining the desired color and brightness in each pixel during one frame interval.
FIG. 1 shows schematically a block diagram for a typical LCD device, which comprises a gray signal modulator 10 for receiving and modulating the input gray signals, a timing controller 11 for controlling the signal sequence and synchronization, a data driver 12 for converting the modulated gray signal to the corresponding voltage data sequence, a gate driver 13 for continuously supplying scanning signals, and a LCD panel 14, comprising a plurality of gate lines 15 for transmitting scanning signals, a plurality of data lines 16 being insulated from and crossing the gate lines 15 for transmitting image signals, and an array of pixels forming by the areas surrounded by said gate lines 15 and said data lines 16.
As can be inferred from FIG. 1, the gray signal modulator 10 plays an important role in the LCD device and the driving circuit thereof. To reduce the problem of afterimage, the original gray signal was first processed by the gray signal modulator 10. The modulated gray signal was then sent into the driving circuit to provide suitable data voltage to each pixel of the LCD device in order to display the desired color and brightness accurately.
FIG. 2 shows a schematic diagram for a conventional gray signal modulator and the operation principle thereof. It comprises an input terminal 20 for receiving gray signals of input images, a frame memory 21 for storing preceding field image data, a frame memory controller 22 for controlling the frame memory 21 and the reading/writing processes therein, a signal converter 23 for modifying the input gray signals, a signal output terminal 24 for sending the modified gray signals to the data driver 12. The main function of the signal converter 23 is to compare the current field image data with the preceding field image data in the frame memory 21 and send out after modifying the output data to a suitable voltage level by compensation voltages. FIGS. 3A and 3B illustrate how the signal modulator modifies the input gray signals. In FIG. 3A, due to the slow response time of the liquid crystal molecules, the output brightness cannot reach the desired brightness during one frame interval. However, as shown in FIG. 3B, after modifying the input gray signals by compensation voltages, the output brightness become able to reach the desired brightness of the source image during one frame interval, thereby the problem of afterimage and blurred image caused by the slow response time can be effectively eliminated. In general, to efficiently process the compensation voltages in the signal converter, a presetting look-up table is commonly used for quick response.
While the technique described above can effectively eliminate the problem of afterimage caused by the slow response time of liquid crystal molecular, however, the noise induced by the gray signal modulator is not taken into account. As can be inferred from FIG. 3, the main function of compensation voltage is to amplify the input gray signal. However, such amplification will also enhance noise, leading to lower signal-to-noise ratio (S/N ratio) and hence lower image quality. On the other hand, the different frame-rate systems are not taken into account in the design of conventional LCD driving method. In fact, when the LCD device is designed for a certain frame-rate system, the response of the liquid crystal molecule during one frame interval will also be different, thereby leading to over (or under) compensation if the frame rate is slower (or faster) than the design. Therefore, to obtain the optimized image quality, the abovementioned problems should be overcome by improving the design of driving method of a LCD device, specifically its design for the preprocessor in the gray signal modulator.